The Molecular Clouds in a Section of the Third Galactic Quadrant: Observational Properties and Chemical Abundance Ratio between CO and its Isotopologues

Abstract

We compare the observational properties between 12CO, 13CO, and C18O and summarize the observational parameters based on a 7069-cloud sample from the Milky Way Imaging Scroll Painting CO survey in a section of the third Galactic quadrant. We find that the 13CO angular area ( A 13 CO ) generally increases with that of 12CO ( A 12 CO ), and the ratio of A 13 CO to A 12 CO is 0.38 by linear fitting. We find that the 12CO and 13CO flux are tightly correlated as F 13 CO = 0.17 F 12 CO with both fluxes calculated within the 13CO-bright region. This indicates that the abundance X 13 CO is a constant as 6.5 − 0.5 + 0.1 × 10 − 7 for all samples under the assumption of local thermodynamic equilibrium (LTE). Additionally, we observed that the X-factor is approximately constant in a large sample of molecular clouds. Similarly, we find F C 18 O = 0.11 F 13 CO with both fluxes calculated within the C18O-bright region, which indicates that the abundance ratio X 13 CO / X C 18 O stays at the same value of 9.7 − 0.8 + 0.6 across the molecular clouds under the LTE assumption. The linear relationships of F 12 CO versus F 13 CO and F 13 CO versus F C 18 O hold not only for the 13CO-bright region and C18O-bright region, but also for the entire molecular cloud scale with a lower flux ratio. The abundance ratio X 13 CO / X C 18 O inside the clouds shows a strong correlation with the column density and temperature. This indicates that the ratio X 13 CO / X C 18 O is dominated by a combination of chemical fractionation, selective dissociation, and the self-shielding effect inside the clouds.

On the Flux–Intensity Relation of Molecular Clouds

“…The means of fractions are about 15.6%, 18.8%, and 16.9%, for 12 CO, 13 CO, and C 18 O, respectively. Figure 6 is in line with the results of Wang et al (2023), who found an excellent correlation between 12 CO and 13 CO emissions, as demonstrated in Figure 12 therein. They derived a value of 0.13 for the ratio of 13 CO to 12 CO emissions, close to the mean flux fraction of the second exponential component revealed by the 12 CO flux-intensity relation.…”

“…Similar to the total flux of molecular clouds, the flux of the second exponential component, F exp 2 , also scales up with the angular area, as demonstrated in panel (c) of Figure 4. F exp 2 occupies about 15.6% (the mean of all double-exponential clouds) of the total flux, very close to the flux ratio (about 13%) of 13 CO/ 12 CO found by Wang et al (2023).…”

Revealing Gas Inflows Toward the Galactic Central Molecular Zone

“…The typical excitation temperature of the gas is 8 ± 1 K (assuming the beam filling factor is 1 for these distant MCs at about 5-8 kpc) according to the peak emission of the 64 large 12 CO structures. More than 70% of samples in the 64 MCs have a relatively small 13 CO-to-12 CO intensity ratio (i.e., I( 13 CO)/I( 12 CO)∼0.1), which is roughly half the value of the normal MCs in the Galactic plane (∼0.2 in previous studies; see e.g., Tokuyama et al 2019;Wang et al 2023). We also find that the closer the MCs in the WGL are to the GC, the greater the velocity dispersion they have.…”

Distribution and Properties of Molecular Gas toward the Monoceros OB1 Region

“…This suggests a connection between high-temperature, high-density gas and star-forming activities in the Mon OB1 region. Actually, some studies also suggest that MCs with high T peak (>10 K, corresponding to T ex >13.4 K, Wang et al 2019Wang et al , 2023b or high column density (e.g., Wang et al 2023b) may be related to possible starforming activities.…”